Testing engine compression can reveal quite a lot about the state of health of your 2.7L engine. Doing an engine compression test will:
- Help diagnose a hard to troubleshoot misfire.
- Help to find out if no compression is behind a ‘cranks but does no start’ condition.
- Tell you the overall health of your engine.
In this tutorial I'll explain how to do and interpret an engine compression test.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2.7L V6 Chrysler, Dodge) (at: autotecnico-online.com).
NOTE: This tutorial applies only to the indicated 2.7L V6 equipped vehicles listed in the Applies To: box on the right column.
Important Tips And Suggestions
TIP 1: To get a more accurate test result, from your compression test, you should perform it with a slightly warmed up engine. I want to emphasize slighly warmed up engine because the engine should not be hot.
TIP 2: Be careful and take all necessary safety precautions as you work around a cranking engine. Your safety is your responsibility, so use common sense and think safety all of the time.
TIP 3: Both cylinder heads are made of aluminum metal, so do not remove the spark plugs with a hot engine. Removing the spark plugs from a hot engine can/will damage the spark plug hole threads in the aluminum cylinder head.
TIP 4: On the rear wheel drive (RWD) 2.7L equipped vehicles you'll need to remove the plastic manifold plenum (also known as the upper intake manifold). Before you start, take a look at the section Precautions To Take When Removing The Intake Manifold Plenum.
Symptoms Of Low Or No Cylinder Compression
Engine compression problems usually fall into one of two categories: Good compression in most but not all 6 cylinders or no compression in all 6 cylinders.
Low engine compression in one or more cylinders will cause a rough idle issue that can seem very hard to diagnose.
No compression in all cylinders results in a ‘cranks but does not start’ condition.
Here are some other specific symptoms you may see with low cylinder compression:
- Misfire codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306.
- Engine won't start (0 compression in all cylinders).
- Engine smokes when running (blue smoke).
- Rough idle (engine misfires).
- Bad gas mileage.
- Engine ‘misses’ at idle but ‘miss’ disappears as you accelerate.
- Check engine light is illuminated with a MAP sensor trouble code (even tho' the MAP sensor is good).
With this info under our belts, let's head down to the next subheading and get testing.
TEST 1: Dry Compression Test
Before you start, let me tell you that the 2.7L V6 engine in your Chrysler/Dodge vehicle suffers from one very common problem. This is valve valve covers leaking and letting oil into the spark plug tubes.
The end result is spark plug wire boots and spark plug drowning in motor oil. This eventually leads to carbon tracks on the side of the spark plugs and on the inside of the plug wire boots and eventually a misfire condition.
So, if you remove a spark plug wire boot or spark plug covered/dripping in oil, you've found a the potential problem behind the misfire you're trying to diagnose with the compression test (you should still proceed with the compression test though).
OK, to get started this is what you'll need to do:
Remove the ignition coils. As you're removing them check to see if the coil boot (also known as the spark plug boot) is covered/dripping motor oil.
Remove the spark plugs from a slightly warmed up engine (if it starts and runs). Remember, the engine can not be hot!
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.
If the engine does not start, don't worry about it being warmed up.
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
Repeat this test step on the remaining cylinders.
Let's take a look at what your test results mean:
CASE 1: If the engine does not start and your engine compression test values for all of the 6 cylinders are 0, you have one of the following conditions:
- Timing chain problem.
- Blown head gasket.
- Blown engine.
Any compression value below 100 PSI (even if it does not 0 PSI) means internal mechanical engine trouble.
CASE 2: If the engine does start, the very first thing you'll notice is that the compression values you wrote down for each cylinder are slightly different from one another. This is normal.
What is NOT normal is if the values vary too much. The cool thing is that we can find out if the variations in the values, you wrote down, indicate a problem (with that cylinder) or not.
The rule of thumb is that they can not vary more than 15% from each other and if they do, you're gonna' have a genuine misfire condition on your hands or possibly a no start condition (if more than one cylinder is affected).
How do you figure this out? This is how you do it:
- Grab a calculator and multiply the highest compression reading that you recorded by .15. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
- Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
- So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
To make better sense of the above calculation, let's say that my 2.7L Dodge Intrepid produced the following compression test results:
- Cylinder #1 160 PSI.
- Cylinder #2 175 PSI.
- Cylinder #3 160 PSI.
- Cylinder #4 110 PSI.
- Cylinder #5 165 PSI.
- Cylinder #6 175 PSI.
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!